Erratic And sudden indicators of mental health issues:

1. Physical health changes that are unexpected and drastic – disease, weight, skin, etc.
2. Difficulty coping with lifestyle changes – residence, work, friends, etc.
3. Sudden obsessions for someone who hasn’t had many. Or the lack of obsessions for someone who usually has them.
4. Need for instant gratification (disproportionate to their normal tendency)
5. Regular reactions (emotional, verbal, physical) are incongruent with normal tendencies
6. Unusual social withdrawal
7. A drop in objectively interpreting events around oneself. 
8. Unexplained mood swings
9. Neglecting responsibilities – professional, personal, social

And of course, the glaringly obvious indicator – someone says ‘I need to work on my mental health.’